Polyester fibers have been the most widely applicable and most productive fiber material in the textile industry. The basic procedures for the manufacture of polyesters are well known, and fibers made from polyesters can be appropriately woven or knitted to form textile fabric. Polyester fibers can be blended with other natural fibers such as wool or cotton to produce textile fibers with enhanced strength and durability. These combined fibers retain not only the physical properties of the polyesters, but also the desired qualities of the natural fibers. Such improved polyester fibers can be processed with other forms of textile fibers to produce final textile fabrics.
In addition, it is known that, if during the manufacture of these synthetic fibers, the polyesters which were initially formed as extruded linear filaments are treated and changed into other shapes, they may exhibit better properties than natural fibers such as wool and cotton. In the art, such treatments are generally referred to as texturizing (improving the texture) and may be comprised of false twisting, crimping and specific chemical treatments. While polyesters exhibit good strength characteristics in a homogeneously polymeric state, the tenacity of polyesters is a typical measurement of their strength, and is expressed as the force (grams) required to break the fabric per denier of a filament. The tenacity of an un-improved polyester filament generally ranges from about 3.5 to about 7 g/denier.
In many applications, it is desirable that the textile fabric be available in a variety of colors, these colors being realized by dyeing. However, since the un-improved polyester is not as stainable as most natural fibers, it must be dyed under the conditions of high temperature, high pressure or both, or at atmospheric conditions with or without using swelling agents, which are commonly referred to as "carriers". Accordingly, various techniques have been developed for enhancing the dyeability of the polyester.
Usually, a special dyeing technique can be employed to enhance the dyeability of polyester fibers. For example, dyestuffs are used to create physical or chemical reactions with polyesters and generally, functional groups of common dye additives are chemically reacted with dye molecules, rather than with the polyester fibers. These functional groups include carboxylic acids (particularly dicarboxylic or other multifunctional acids), organometallic sulfate or sulfonate compounds and polyethylene glycol. While Toga et. al. discloses in U.S. Pat. No. 4,415,727 the use of 2-methyl-1,3-propanediol as a crystallization-controlling agent for PET (polyethylene terephthalate) bottle molding materials, he fails to mention the function of this substance in improving the dyeability, strength and elongation of the fiber.